A trophic cascade occurs when a change at one level of a food web propagates down through the levels beneath it. The mechanism is real and demonstrated. The most famous example of it, the wolves of Yellowstone, has become considerably more disputed than its popular retelling suggests.

Remove a top predator and its prey increase, which suppresses whatever the prey eats. Restore the predator and the sequence reverses. The effect can operate through consumption, by reducing prey numbers, or through fear, by changing where prey feel safe to feed, which is called a landscape of fear.

The clearest demonstrations come from aquatic systems, where the experiments are tractable. Removing otters produces urchin barrens; removing large fish from lakes changes zooplankton and then algae in predictable ways. These results are not in dispute.

A kelp forest. Marine systems provide the cleanest evidence for cascades, because the levels are distinct and the experiments can be run at meaningful scales.
A kelp forest. Marine systems provide the cleanest evidence for cascades, because the levels are distinct and the experiments can be run at meaningful scales.Credit: User:Mas3cf (CC BY-SA 4.0).

The popular account runs like this: wolves were exterminated from Yellowstone by 1926, elk multiplied and overgrazed willow and aspen along the rivers, and when wolves were reintroduced in 1995 the elk moved away from valleys, the trees recovered, beavers returned, riverbanks stabilised, and the rivers physically changed course. A short film built on this account has been watched many tens of millions of times.

Considerable subsequent research disputes most of the chain.

Elk numbers did fall, but the reintroduction coincided with a severe drought, increased hunting outside the park, and growing grizzly and cougar populations, so attributing the decline to wolves alone is not supportable. Willow recovery has been patchy rather than general, and several studies find it limited by water table and beaver activity rather than by browsing pressure. The landscape-of-fear effect, that elk avoid risky areas, has been tested repeatedly with mixed results, and some careful studies find little behavioural change. The claim that rivers changed course is the weakest link and is not supported by the geomorphological work.

A wolf. The Yellowstone reintroduction of 1995 is the most cited trophic cascade in the world, and most links in its popular retelling are disputed in the research literature.
A wolf. The Yellowstone reintroduction of 1995 is the most cited trophic cascade in the world, and most links in its popular retelling are disputed in the research literature.Credit: w:en:Aquaimages (talk | contribs) (CC BY-SA 2.5).

Almost no ecologist denies that wolves have ecological effects. The disagreement is about magnitude, mechanism, and how confidently a single causal chain can be extracted from an uncontrolled system where many things changed at once.

The episode is worth understanding beyond ecology, because it shows how a compelling narrative outruns its evidence. The story is elegant, it is emotionally satisfying, it supports conclusions many people already hold about predators and wilderness, and those qualities are exactly why it spread faster than the qualifications did. Researchers who question the details are frequently assumed to oppose wolf reintroduction, which is a separate question and mostly not their position.

Cascades are real. Yellowstone is a weaker case for them than its fame implies, and the strongest evidence remains in marine and freshwater systems where the experiment can actually be run.